fix(ui): failureReason "usage_limit" nachziehen + Worktree-Gate prüft die Platte

Zwei liegengebliebene Consumer aus den letzten beiden Commits:

- TaskRunner klassifiziert seit 07dd7570 "usage_limit", aber weder
  TaskRowViewModel.FailureReasonLabel noch vm.failureReason (de/en) noch
  die get_task-Tool-Beschreibung kannten den Wert — die UI zeigte
  "Grund unbekannt", das MCP-Doc listete weiterhin max_turns|timeout|error.
- TaskRowViewModel.CanOpenWorktree prüfte nur auf einen nicht-leeren
  String. Die Zeile behält den Path eines gemergten/verworfenen Worktrees,
  also war der Menüpunkt aktiv und Process.Start warf in den Footer.
  Jetzt zusätzlich Directory.Exists — dieselbe Prüfung, die
  WorktreesOverviewModalViewModel und MergeSectionViewModel schon machen.

src/ClaudeDo.Localization/locales/de.json

@@ -719,7 +719,7 @@
     "mainWindow": { "closeConfirm": { "message": "{0} Mission-Control-Sitzung(en) laufen noch. Beim Beenden werden sie abgebrochen. Trotzdem schließen?" } },
     "agentStatus": { "idle": "Leerlauf", "queued": "In Warteschlange", "running": "Läuft", "review": "Prüfung", "children": "Wartet auf Teilaufgaben", "done": "Fertig", "failed": "Fehlgeschlagen", "cancelled": "Abgebrochen" },
     "taskStatus": { "idle": "Leerlauf", "queued": "In Warteschlange", "running": "Läuft", "waitingForReview": "Wartet auf Prüfung", "waitingForChildren": "Wartet auf Teilaufgaben", "done": "Fertig", "failed": "Fehlgeschlagen", "cancelled": "Abgebrochen", "parked": "Geparkt", "interactive": "Interaktiv" },
-    "failureReason": { "maxTurns": "Turn-Limit erreicht", "timeout": "Zeitüberschreitung", "error": "Fehler", "cancelled": "Abgebrochen", "unknown": "Grund unbekannt" },
+    "failureReason": { "maxTurns": "Turn-Limit erreicht", "timeout": "Zeitüberschreitung", "usageLimit": "Nutzungslimit erreicht", "error": "Fehler", "cancelled": "Abgebrochen", "unknown": "Grund unbekannt" },
     "failureReasonTooltip": { "maxTurns": "Turn-Limit erreicht ({0}/{1} Turns) — der Worktree ist meist brauchbar; Task fortsetzen statt zurücksetzen." },
     "planningBadge": { "active": "PLANUNG", "finalized": "GEPLANT" },
     "taskRow": { "createdPrefix": "Erstellt {0}", "stepsText": "{0}/{1} Schritte" },

src/ClaudeDo.Localization/locales/en.json

@@ -719,7 +719,7 @@
     "mainWindow": { "closeConfirm": { "message": "{0} Mission Control session(s) are still running. Closing the app will end them. Close anyway?" } },
     "agentStatus": { "idle": "Idle", "queued": "Queued", "running": "Running", "review": "Review", "children": "Waiting for Subtasks", "done": "Done", "failed": "Failed", "cancelled": "Cancelled" },
     "taskStatus": { "idle": "Idle", "queued": "Queued", "running": "Running", "waitingForReview": "Waiting for Review", "waitingForChildren": "Waiting for Subtasks", "done": "Done", "failed": "Failed", "cancelled": "Cancelled", "parked": "Parked", "interactive": "Interactive" },
-    "failureReason": { "maxTurns": "Turn limit reached", "timeout": "Timed out", "error": "Error", "cancelled": "Cancelled", "unknown": "Unknown reason" },
+    "failureReason": { "maxTurns": "Turn limit reached", "timeout": "Timed out", "usageLimit": "Usage limit reached", "error": "Error", "cancelled": "Cancelled", "unknown": "Unknown reason" },
     "failureReasonTooltip": { "maxTurns": "Turn limit reached ({0}/{1} turns) — the worktree is usually fine; continue the task instead of resetting it." },
     "planningBadge": { "active": "PLANNING", "finalized": "PLANNED" },
     "taskRow": { "createdPrefix": "Created {0}", "stepsText": "{0}/{1} steps" },

src/ClaudeDo.Ui/ViewModels/Islands/TaskRowViewModel.cs

@@ -180,7 +180,11 @@ public sealed partial class TaskRowViewModel : ViewModelBase
     // placeholder so the menu wiring is uniform; always null until a real gate exists.
     public string? QuickSessionDisabledReason => null;
 
-    public bool CanOpenWorktree => !string.IsNullOrWhiteSpace(WorktreePath);
+    // The row keeps a merged/discarded worktree's recorded Path, so a non-empty string is not
+    // proof the folder is still there — same on-disk check the worktrees overview and the merge
+    // section do before offering "open". Only read when the context menu is built, not per row.
+    public bool CanOpenWorktree =>
+        !string.IsNullOrWhiteSpace(WorktreePath) && System.IO.Directory.Exists(WorktreePath);
     public string? OpenWorktreeDisabledReason => CanOpenWorktree ? null : Loc.T("tasks.reasonNoWorktree");
 
     public string? RefineDisabledReason
@@ -235,6 +239,7 @@ public sealed partial class TaskRowViewModel : ViewModelBase
     {
         "max_turns"   => Loc.T("vm.failureReason.maxTurns"),
         "timeout"     => Loc.T("vm.failureReason.timeout"),
+        "usage_limit" => Loc.T("vm.failureReason.usageLimit"),
         "cancelled"   => Loc.T("vm.failureReason.cancelled"),
         "error"       => Loc.T("vm.failureReason.error"),
         _             => Loc.T("vm.failureReason.unknown"),

src/ClaudeDo.Worker/External/ExternalMcpService.cs

@@ -333,7 +333,7 @@ public sealed class ExternalMcpService
         "Done/Failed/Cancelled tasks can be reset to Idle for re-execution. A Queued task with a blocker waits " +
         "for its predecessor before the picker will claim it, and WaitingForChildren is a parent whose own work " +
         "is done but whose children are still running. For Status=Failed, failureReason (max_turns|timeout|" +
-        "error|cancelled|unknown) plus failureTurnsUsed/failureMaxTurns say why without pulling get_task_log." +
+        "usage_limit|error|cancelled|unknown) plus failureTurnsUsed/failureMaxTurns say why without pulling get_task_log." +
         McpToolDocs.TaskNumberHint)]
     public async Task<TaskDto> GetTask(string taskId, CancellationToken cancellationToken)
     {

tests/ClaudeDo.Worker.Tests/UiVm/TaskRowViewModelTests.cs

@@ -65,14 +65,20 @@ public class TaskRowViewModelTests
     }
 
     [Fact]
-    public void CanOpenWorktree_True_And_ReasonNull_When_Path_Present()
+    public void CanOpenWorktree_True_And_ReasonNull_When_Folder_Exists()
+    {
+        var dir = Path.Combine(Path.GetTempPath(), $"cd_wt_{Guid.NewGuid():N}");
+        Directory.CreateDirectory(dir);
+        try
         {
             var vm = new TaskRowViewModel { Id = "t" };
-        vm.WorktreePath = @"C:\repo\worktrees\t";
+            vm.WorktreePath = dir;
 
             Assert.True(vm.CanOpenWorktree);
             Assert.Null(vm.OpenWorktreeDisabledReason);
         }
+        finally { Directory.Delete(dir, true); }
+    }
 
     [Theory]
     [InlineData(null)]
@@ -87,6 +93,18 @@ public class TaskRowViewModelTests
         Assert.NotNull(vm.OpenWorktreeDisabledReason);
     }
 
+    // A merged/discarded worktree leaves its recorded Path on the row long after the folder is
+    // gone — the menu entry has to gray out, not throw inside Process.Start.
+    [Fact]
+    public void CanOpenWorktree_False_When_Recorded_Folder_Is_Gone()
+    {
+        var vm = new TaskRowViewModel { Id = "t" };
+        vm.WorktreePath = Path.Combine(Path.GetTempPath(), $"cd_wt_gone_{Guid.NewGuid():N}");
+
+        Assert.False(vm.CanOpenWorktree);
+        Assert.NotNull(vm.OpenWorktreeDisabledReason);
+    }
+
     [Fact]
     public void OpenWorktreeGate_Raises_PropertyChanged_On_WorktreePath_Change()
     {
@@ -115,7 +133,6 @@ public class TaskRowViewModelTests
         var vm = TaskRowViewModel.FromEntity(entity);
 
         Assert.Equal(@"C:\repo\worktrees\t", vm.WorktreePath);
-        Assert.True(vm.CanOpenWorktree);
     }
 
     [Fact]